1. Field of the Invention
The present invention relates to a photovoltaic element including a solar cell, a process for producing said photovoltaic element, a method for removing a cover portion of a covered wire, and a method for joining a covered wire and a conductor. More particularly, the present invention relates to a photovoltaic element having an improved electrode structure, and a process for producing said photovoltaic element.
2. Related Background Art
In recent years, public attention has been focusing on a sunlight power generation system. Along with this, there is an increased demand for providing a large area photovoltaic element at a reasonable cost which enables one to establish a desirable sunlight power generation system at a reasonable cost. In general, a photovoltaic element has a photovoltaic layer (comprising a photoelectric conversion semiconductor layer), and in the photovoltaic element, the resistivity of a charge in the photovoltaic layer against its migration in an elemental face direction is large. Because of this, in the case of a large area photovoltaic element, in order to diminish a joule loss due to the migration of the charge in the elemental face direction, an electrode comprising a metal (this electrode will be hereinafter referred to as metal electrode) which has high conductivity is provided on the surface of the photovoltaic element.
In the case where the metal electrode is provided on the side where light is impinged, because the metal electrode is generally opaque, it is formed such that it does not shut out incident light as much as possible. On the other hand, in the case where the metal electrode is provided on the side where no light is impinged, it may be formed on the entire surface involved but in view of reducing the cost, it is often formed at a minimum necessary portion thereof.
Now, for instance as shown in FIGS. 7(a) and 7(b) it is known that an electrode 700 is formed in a comb-shaped form on a light incident side face or a back side face of a photovoltaic element (not shown). FIG. 7(a) is a schematic plan view of said electrode, and FIG. 7(b) is a schematic cross-sectional view taken along the line H-Hxe2x80x2 in FIG. 7(a).
In the case where the electrode 700 is formed on the light incident side face, when the electrode 700 comprises a comb-shaped electrode comprising a plurality of wire electrodes being arranged at a prescribed equal interval as shown in FIG. 7(a), it is possible to effectively collect charges while restraining the joule loss. In the case where the electrode is formed on the back side face, it can be formed at a low cost because it can be formed using an electrode-forming material in a small amount. However, the formation of such electrode is generally conducted by a method wherein an electrically conductive resin material 702 is screen-printed on the surface of the photovoltaic element, followed by subjecting to sintering treatment, and a solder paste 701 is screen-printed thereon, followed by subjecting to reflow soldering. However, in this method, it is difficult to thicken the thickness of the electrode, and therefore, in the case where the photovoltaic element involved is of a larger area, it is difficult to attain an effect of sufficiently diminishing the joule loss.
In order to improve this situation, Japanese Unexamined Patent Publication No. 36395/1997 discloses a configuration for a comb-shaped electrode as shown in FIGS. 8(a) and 8(b) in that a core wire 801 whose thickness being capable of being thinned and an electrode 802 are used. FIG. 8(a) is a schematic plan view of said comb-shaped electrode, and FIG. 8(b) is a schematic cross-sectional view taken along the line I-Ixe2x80x2 in FIG. 8(a). Particularly, the comb-shaped electrode shown in FIGS. 8(a) and 8(b) comprises a covered wire comprising a metal core wire 801 covered by an electrically conductive resin material 803 which is bonded onto the surface of a photovoltaic element (not shown) by way of thermocompression bonding. And in said publication, there is described that the covered wire is joined with an electrode 802 through a metal layer comprising a solder or the like or an electrically conductive resin material.
Incidentally, for the conventional comb-shaped electrode in which the core wire and the electrode having such configuration as above described, it is difficult to make the comb-shaped electrode to have a sufficient resistance to stress. For instance, in the case where the core wire is joined with the electrode only through a metal layer by way soldering, although the joining may be conducted in a low resistance state, the joined portion is poor in flexibility and therefore, there is a problem in that the stress added to the core wire is converged to the joint of the joining portion of the core wire, where the core wire is liable to readily fracture at the joint thereof. In the case of a solar cell module in which such photovoltaic element is used, there is a tendency in that the above situation lead to entailing such problems as will be described in the following. That is, due to the stress remained at the core wire and a load applied onto the solar cell module by virtue of wind or snow, the solar cell module is flexed, where stress is eventually applied to the core wire and further stress is applied to the core wire when the temperature of the photovoltaic element is changed and as a result, the core wire is ruptured at the joint of thereof, resulting in a decrease in the energy conversion efficiency (the photoelectric conversion efficiency) of the solar cell module.
Separately, in the case where the core wire is joined with the electrode only through the electrically conductive resin material which comprises a composition comprising electrically conductive fine particles dispersed in a given resin, a current flow passage comprising said electrically conductive fine particles is formed between the core wire and the electrode, where gaps among the electrically conductive fine particles are filled with the resin to retain the current flow passage. In this case, it is also difficult to make the comb-shaped electrode to have a sufficient resistance to stress. That is, when it is intended to increase the current flow passage so as to lower the resistance, it is necessary to increase the content ratio of the electrically conductive fine particles in the electrically conductive resin material. In this case, the content ratio of the resin which serves to retain the current flow passage is decreased and as a result, the flexibility of the joining portion is diminished, where a sufficient resistance to stress cannot be attained. Therefore, there is also a tendency in that such problems as above described are entailed.
In the following, description will be made of a covered wire.
The covered wire means a composite comprising a core wire covered by a coating material. There are known various covered wires such configured, which are used in various fields. For instance there are known covered wires whose core wire comprising a wire for transmitting light such as an optical fiber which are used in the field of optical instrument; covered wires whose core wire comprising a wire for transmitting heat which are used in the field of refrigeration and also in the field of temperature measurement; and covered wires whose core wire comprising a wire for transmitting electromagnetic field which are used in the field of electronics. Particularly, in the field of solar cells, it is a common technique that a covered wire comprising an electrically conductive core wire covered by an electrically conductive resin material is disposed on the surface of photovoltaic element in order to collect power generated by the photovoltaic element.
The cover of the covered wire is generally used for the purpose of preventing a thing to be transmitted by the core wire from leaking to the outside or for the purpose of releasing said thing while adjusting the quantity thereof. However, there is a case in that the covered wire is preferred to be made such that it has an exposed portion with no coat. As such case, there can be mentioned, for example, a case in that a covered wire whose core wire comprising an optical fiber is made to have an exposed portion with no coat so that light leaked through said exposed portion can be received by a detector, and a case in that a covered wire whose core wire comprising a copper wire is made to have an exposed portion with no coat and said exposed portion is used as a switch of making ON or OFF by attaching a connector to or detaching said connector from said exposed portion.
The formation of a covered wire having such exposed portion can be conducted by a method of forming a covering coat on a given core wire except for a prescribed portion of said core wire. However, said covered wire can be readily formed by a method of forming a covering coat over the entire surface of a given core wire and removing a prescribed portion of the covering coat on the core wire by radiating energy beam such as laser beam to establish an exposed portion at said covered wire.
Japanese Unexamined Patent Publication No. 7825/1995 (hereinafter referred to as document 1) disclose a technique in that a covered wire comprising a gold wire or a gold-plated wire as a core wire covered by a covering material comprising an insulating resin material is extended in the air and excimer laser beam is radiated to a prescribed portion of the covered wire to remove the corresponding portion of the cover of the covered wire to make the covered wire have an exposed wire potion.
Japanese Unexamined Patent Publication No. 174263/1997 (hereinafter referred to as document 2) discloses a technique in that YAG laser beam is radiated to a prescribed portion of a covered wire whose cover comprising an insulating material which is wound on a terminal to remove the corresponding portion of the cover of the covered wire to make the covered wire have an wire exposed portion.
Incidentally, there are known various structures comprising a covered wire having a portion with no cover (an exposed wire portion) and a separate component (capable of transmitting a thing which is transmitted by the core wire of the covered wire) joined with said exposed wire portion of the covered wire, which are used in various fields. Particularly, in the field of electronics, a covered wire comprising a core wire covered by a given covering material has been widely used. And it is a common technique that a prescribed portion of the cover of the covered wire is removed to make the covered wire have an exposed wire portion, and said exposed wire portion of the covered wire is joined with another electrode. In this case, the thing which is transmitted from the core wire to said another electrode is electricity. For instance, in document 1, there is described that the exposed wire portion of the covered wire is connected to other electronics component by way of wire-bonding. In document 2, there is described that a solder is supplied to the exposed wire portion of the covered wire to join the covered wire with the terminal.
In the prior art, there are such shortcomings as will be described below, in terms of insufficiency in removal of the cover of a covered wire.
In the technique described in document 1, the covered wire is extended in the air and excimer laser beam is radiated to a prescribed portion of the covered wire as previously described. In this case, there is a tendency in that freedom occurs at the position of the covered wire, where when the laser beam is radiated, an action for the covered wire to escape from the layer exerts. This situation makes it difficult to sufficiently remove a prescribed portion of the cover of the covered wire. In the technique described in document 2, YAG laser beam is radiated to a prescribed portion of the covered wire would on the terminal as previously described. In this case, there is a tendency in that the covered wire absorbs energy of the laser beam to thermally expand, where the covered wire wound on the terminal is loosened to occur freedom at the position of the covered wire and when the laser beam is radiated, an action for the covered wire to escape from the layer exerts. This situation makes it difficult to sufficiently remove a prescribed portion of the cover of the covered wire.
In addition according to such technique in the prior art, it is difficult to readily join the cover-removed core wire portion (the exposed core wire portion) of the covered wire with the conductor (the electrode) so that the joining between the two members is sufficient in terms of the reliability.
Now, in order to attain highly reliable joining between given two different members by a joining manner by means of brazing using a solder or the like, a joining manner using an adhesive such as an electrically conductive paste, a joining manner by means of welding, or a joining manner by means of ultrasonic bonding, it is required that the two members are brought to adequately come closer to each other.
In the technique described in document 1, it is difficult that the cover-removed portion of the covered wire is made to be in contact with other electronics component. When the laser beam is radiated to the covered wire simply from one direction, a face of the covered wire which is opposed to the face thereof to which the laser beam is radiated is shaded from the radiation of the laser beam, where the covering material of said face tends to remain without being removed, and thus, cover removal is substantially performed only for the face to which the laser beam is radiated. In this connection, in order to contact the cover-removed portion of the covered wire with the electronics component, it is necessary to adopt a covered wire-handling mechanism that the face of the exposed core wire which is provided as a result of a portion of the cover situated at said face being removed by the radiation of the laser bean can be precisely contacted with the electronics component. However, the covered wire is in a fine line form in many cases and is, therefore, liable to readily twist or bend. In addition, the cover-removed portion of the covered wire is of a tiny area. Because of this, such handling mechanism is difficult to be realized. In this respect, in document 1, there is adopted a mechanism that without simply radiating the laser beam to the covered wire, cover removal is conducted for all around the covered wire. Particularly, there is adopted a mechanism that using a concave mirror, the laser beam is radiated also to the face of the covered wire which is opposed to the face thereof to which the laser beam is directly radiated. However, for this mechanism, there is a shortcoming in that when the concave mirror has staining or the like, the laser beam-radiating processing becomes unstable, there sufficient cover removal cannot be performed as desired.
In the technique described in document 2, because the covered wire is wound on the electronics component with which the covered wire is joined, it is difficult to irradiate the laser beam to a face of the covered wire which is faced to the electronics component as desired and therefore, it is difficult to sufficiently remove a portion of the cover situated at said face. In this connection, the cover of the covered wire is partially present between the cover-removed portion (the exposed core wire potion) and the electronics component, where a residual portion of the cover is present such that it is sandwiched between the two member to be joined with each other, and because of this, the two members cannot be sufficiently connected with each other. In this connection, in the case of the joining method wherein an adhesive such as a solder or an electrically conductive paste is made to present between the exposed core wire portion of the covered wire and the electronics component, it is necessary to form a conduction path by the solder or the electrically conductive paste so as to detour the residue of the cover present between the exposed core wire and the electronics component. The joining thus established is liable to be poor in terms of the reliability. Besides, in the case where the joining method of directly joining the exposed core wire portion of the covered wire with the electronics component by way of welding or ultrasonic bonding is adopted, the joining between the two members cannot be performed as desired because the residue of the cover is present between the two members.
In view of the foregoing situation in the prior art, the present invention makes it a first object to provide a photovoltaic element having an highly reliable electrode comprising a covered wire having a core wire which makes it possible to desirably maintain a good energy conversion efficiency (photoelectric conversion efficiency) of a solar cell module formed using said photovoltaic element, and a process for producing said photovoltaic element.
A typical embodiment of the photovoltaic element provided according to the present invention comprises a photovoltaic element, a core wire disposed on the surface of said photovoltaic element for outputting a power generated by said photovoltaic element, and an electrode electrically joined with said core wire while forming a joining portion, wherein said joining portion of said core wire and said electrode has at least a first joining portion and a second joining portion which is adjacent to said first joining portion, and said second joining portion has an elasticity which is greater than that of said first joining portion.
The process according to the present invention includes the following two embodiments.
A first embodiment is a process for producing a photovoltaic element comprising a photovoltaic element, a wire disposed on the surface of said photovoltaic element for outputting a power generated by said photovoltaic element, and an electrode electrically joined with said wire, wherein at least two different joining portions having a different elasticity are formed between said wire and said electrode.
A second embodiment is a process for producing a photovoltaic element comprising a photovoltaic element, a covered wire whose core comprising a wire disposed on the surface of said photovoltaic element for outputting a power generated by said photovoltaic element, and an electrode electrically joined with said wire of said covered wire, said process comprising the steps of (a) removing part of the cover of said covered wire to form a cover-free portion at said covered wire, (b) fixing said covered wire on the surface of said photovoltaic element, and (c) joining said wire of said covered wire and said electrode each other at a position (i) of said cover-free portion of said covered wire and at a desired position (i) of the remaining portion of said covered wire.
Of the two jointing portions formed between the covered wire and the electrode, it is preferred to make such that the joining portion formed at the position (ii) has an elasticity which is greater than that of the joining portion formed at the position (i). Further, it is preferred that the wire is covered by a covering material comprising an electrically conductive resin material, the wire and the surface of the photovoltaic element are electrically connected through said electrically conductive resin material, and a portion of said electrically conductive resin material which is situated between the wire and the electrode functions as the foregoing second joining portion. It is also preferred that prior to conducting the step (a) of removing part of the cover of the covered wire, the step (b) of fixing the covered wire on the surface of the photovoltaic element is conducted. Further, it is preferred that the step of removing part of the cover of the covered wire is conducted by means of radiation of laser beam.
In addition, it is also preferred that the foregoing first joining portion comprises a metal and the foregoing second joining portion comprises an electrically conductive resin material.
The present invention makes it a second object to attain the followings.
(1) An adequate cover-removing method which can be desirably adopted in a process of removing a cover portion of a covered wire having a core wire by radiation of energy beam, said method being capable of performing cover removal of the covered wire while preventing the covered wire from being positionally deviated.
(2) An adequate cover-removing method which can be desirably adopted in a process of removing a cover portion of a covered wire having a core wire by radiation of energy beam and joining a conductor (an electrode) with the cover-removed portion of said covered wire, said method being capable of readily and surely contacting the core wire of the covered wire with the conductor (the electrode).
The cover-removing method according to the present invention includes the following embodiments.
A first embodiment is a cover-removing method comprising the steps of fixing at least part of a covered wire having a core wire onto a substrate though a fixing member and radiating energy beam to at least part of said fixed covered wire to remove a desired cover portion of the covered wire.
A second embodiment is a cover-removing method comprising the steps of fixing at least part of a covered wire having a core wire onto a substrate though a fixing member, radiating energy beam to at least part of said fixed covered wire to remove a desired cover portion of the covered wire, and joining a conductor (an electrode) with said cover-removed portion of the covered wire and also with a cover portion of the covered wire which is adjacent to said cover-removed portion.
It is preferred for the fixing member to comprise an adhesive material or a double-coated adhesive tape. It is preferred that the fixing member has an energy absorption factor against the energy beam which has a ratio of 0.0001 to 0.9999 versus that of the cover of the covered wire.
It is possible that the substrate comprises a photovoltaic element, the covered wire comprises a collecting electrode of said photovoltaic element, and the conductor comprises a bus bar electrode.